A New Spin on Earth's Rotation

by Michael Schirber | February 25, 2005 02:00am ET

Credit:

We all know that money makes
the world go round, but what causes it to change speed ever so slightly? The
Earth is known to constantly slow down and speed up, which imperceptibly alters
the length of our days.

"We are taught that
the day is 24 hours, or 86,400 seconds. That is true, but there are
irregularities on the millisecond level," said David Salstein, an
atmospheric scientist at Atmospheric and Environmental Research, Inc.

A Russian scientist claimed
this week that these irregularities can be used to predict weather and earthquakes.

After long-term
observations of tidal oscillations of the Earth's rotation speed and
atmospheric processes, N.S. Sidorenkov from the Hydrometeorological Center of
Russia has devised a model that can supposedly forecast the weather from
rotation data more accurately than current meteorological methods.

"There certainly is an
association between weather and rotation," Salstein told LiveScience
in a telephone interview.

But Salstein is of the mind
that changes in the weather drive the rotational irregularities, not the other
way around. He admitted, though, that proving what is the cause and what is the
effect is not easy.

At the whim of the wind

The Earth's rotation speed
can be measured using different techniques. One of the oldest ways is with
multiple radio telescopes all observing the same very distant object. The
telescope signals can be combined to give a precise position of the Earth.

More recently, global
positioning satellites (GPS) and laser ranging (bouncing a laser beam off of
the Moon or a satellite) can also determine the speed with which our planet
spins on its axis.

Make
that Leap

When changes
in the length of the day accumulate, it becomes necessary
to add a leap second. The organization responsible for keeping
track of this is the International Earth Rotation Service
(IERS). Since 1972, the IERS has added 22 leap seconds.
The last leap second was on Dec. 31, 1998. Learn
More

The rotation data shows
oscillations over several different timescales. The one with the largest
variation is seasonal: Earth slows down in January and February.

"It turns out that
during the Northern Hemisphere winter, the winds - which are predominantly west
to east - are stronger," Salstein said.

The more forceful winds
double the angular momentum of the atmosphere. Angular momentum is a conserved
quantity in nature - the example usually given is the spinning ice skater
conserving angular momentum by speeding up when she brings her arms in.

In this case, the whole
system - comprised of the spinning Earth and swirling atmosphere - adjusts to
the blustery winter months by slowing down the solid Earth's rotation. This
means the days get longer - by a few thousandths of a second.

The winter in the Southern
Hemisphere does not produce the same increase in wind because the bottom of the
world is mostly ocean and the temperature swings are, therefore, not as great.

Other associations

There are other smaller
oscillations besides the seasonal one. The Madden-Julian cycle, which is a variation
in tropical weather patterns that lasts about 30 to 60 days, corresponds to
changes in the Earth's rotation.

On longer scales, the
length of the day increases slightly during an El Nino - the two- to four-year
cycle in Pacific Ocean temperatures that fuels global climate change. This is
due to a shift in the Northern Hemisphere's jet stream, which is a
high-altitude, fast-moving wind current.

Some have suggested that global warming will slow the planet, but Salstein said "nothing
definitive has yet been seen." He believes the warming by itself will not
change the rotation speed, unless the poles warm faster than the equator - as
some models do predict.

Climate and weather may not
be the only phenomena related to rotation. There are indications that changes
in the interior of the Earth may alter the spinning speed, as well as the
position of the poles. Some scientists claim that December's tsunami-causing Sumatra earthquake resulted in a
shortening of the day
by 2.68 millionths of a second.

Moreover, the Moon and the
Earth are slowly drifting away from each other. To conserve angular momentum, the
Earth is slowing down, and the day is lengthening at a rate of about a
millisecond per century.

Why it matters

Because the dominant
relationship in Salstein's observations is between wind and rotation, he
attempts to predict the Earth's rotation from long-range weather forecasts.

This rotation forecast is
important to NASA's Jet Propulsion Laboratory because they need an exact
reference to send out navigation signals to their spacecraft in the far reaches
of the solar system. Although a millisecond difference in the length of the day
seems like a small change, "at the radius of Jupiter that will be
important," Salstein said.

It may not be so simple to
say if weather is causing changes in the rotation, as Salstein presupposes, or
if rotation drives shifts in climate, as Sidorenkov claims. Although weather
might seem to be immune to millisecond changes in the Earth's rotation, it
should be remembered that the Earth's rotation does play an important part in
creating general weather patterns.

For instance, the motion of
the jet stream and the counterclockwise-moving winds around low-pressure
systems in the Northern Hemisphere (opposite in the Southern) are manifestations
of the Earth turning underneath the atmosphere.

It may be that rotation and
weather are so intricately tied that it may not make sense to speak of cause
and effect. The Earth slowing down may drive a change in wind patterns, which causes
another alteration to the rotation, which then kicks up more wind, ad
infinitum.

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Author Bio

Michael Schirber

Michael Schirber began writing for LiveScience in 2004 when both he and the site were just getting started. He's covered a wide range of topics for LiveScience from the origin of life to the physics of Nascar driving, and he authored a long series of articles about environmental technology. Over the years, he has also written for Science, Physics World, andNew Scientist. More details on his website.